Conventionally, railroad vehicles such as a Shinkansen bullet train are required to travel at a higher speed and to have the maximum possible transportation capacity. This necessitates a reduction in size and weight of the vehicle body and its ancillary devices, whereas the vehicle-mounted transformer that is particularly large in mass among other ancillary devices has been increased in capacity.
In recent years, there are increasing demands for a low-floor vehicle for the purpose of achieving a barrier-free design. Accordingly, with regard to the underfloor device such as a vehicle-mounted transformer disposed under the floor of the vehicle such as an alternating-current (AC) electric train, there is not only a demand for a reduction in size and weight, but also a strong demand to lower the height of the device for the purpose of achieving a lower-floor vehicle.
For example, Japanese Patent Laying-Open No. 9-134823 (Patent Document 1) discloses a core-type vehicle-mounted transformer as described below. This core-type vehicle-mounted transformer that is cooled by a oil-feeding/air-cooling mechanism is configured to have an inner structure in which a low-voltage winding is wound around the outer periphery of a leg of the iron core and a high-voltage winding is wound around the outer periphery of the low-voltage winding while a cooling oil path is formed between the windings. The inner structure is disposed within a tank such that the above-described cooling oil path extends in parallel to the bottom of the tank. Furthermore, the iron core has two legs, and the low-voltage and high-voltage windings each are divided and wound around the corresponding one of the legs. Since each of the windings is divided into two pieces, the capacity of each winding is reduced by half. As the winding conductor is decreased in size accordingly, the size of one winding in the radial direction is decreased. Consequently, the transformer can be entirely decreased in height, and thus, can be decreased in size.    Patent Document 1: Japanese Patent Laying-Open No. 9-134823